1. Field of the Invention
This invention relates to a magnetooptical recording medium capable of being overwritten by light modulation, and a recording method and apparatus using the medium.
2. Related Background Art
As techniques for overwriting magnetooptical recording media in the form of exchange coupled laminated magnetic films, the light modulation method and the magnetic field modulation method are known. The light modulation method is superior to the magnetic field modulation method in terms of the capability of high-speed modulation or the capability of using both sides of the medium and so on. Such a light modulation method is shown in Japanese laid open patents JP-A-62-175948, JP-A-63-52354 and JP-A-63-153752.
The basic process of overwriting by the light modulation method using exchange coupled laminated films is as follows. The exchange coupled laminated films usually basically comprise a first magnetic layer (memory layer) and a second magnetic layer (writing layer). The first magnetic layer comprises a material which has a relatively high coercivity at room temperature, and has a relatively low Curie temperature. On the other hand, the second magnetic layer comprises a material which has a relatively low coercivity at room temperature, and a relatively high Curie temperature. Between the two layers, at the interface, exchange coupling occurs. After irradiation by a laser during the cooling down period, the information bits (domains) recorded in the second magnetic layer are transferred to the first magnetic layer.
The laser irradiation is modulated to have two different laser powers to record the information. The first laser power heats the medium to a high temperature to record information bits in the second magnetic layer. The other laser power heats the medium to a low temperature to transfer the information bits of the second magnetic layer to the first magnetic layer.
After the laser irradiation, the temperature of the medium reduces to room temperature, and the required magnetic filed is applied to initialize the second magnetic layer. The information bits transferred to the first magnetic layer are not erased by the initializing magnetic field, because of the high coercivity of the first magnetic layer at room temperature. As a result, regardless of the condition of the first magnetic layer before the recording, newly recorded information in the second magnetic layer is usually recorded in the first layer and retained there to obtain overwriting.
However, the method described above requires means for the application of an initializing magnetic field. Therefore, the inconvenience of a large and complex recording apparatus occurs.
Some recording media designed to resolve the inconvenience described above are shown in JP-A-63-268103, JP-A-3-219449, or WO90/02400 and so on.
These improved media comprise laminated structure magnetooptical recording media to which has been added a third magnetic layer (switching layer) and a fourth magnetic layer (initializing layer) over the second magnetic layer of the exchange coupled laminated films. The third magnetic layer (switching layer) has a Curie temperature which is lower than that of the first magnetic layer and higher than room temperature. The fourth magnetic layer (initializing layer) has a Curie temperature which is higher than that of the second magnetic layer.
In these improved media, the fourth layer is previously initialized. After irradiation by the laser, while the temperature of the magnetooptic recording medium cools down to the room temperature, once the temperature of the optical magnetic recording medium is less than the Curie temperature of the third magnetic layer, an exchange coupling force operates between the second magnetic layer and the fourth magnetic layer and the second magnetic layer is initialized. Otherwise, the overwriting is obtained much the same as the process described before. However, such a medium also needs the application of a recording bias magnetic field for recording the second magnetic layer. There are thus some difficulties in making the recording apparatus smaller, simpler and cheaper.
Therefore, some media which reduced the recording bias magnetic field or make unnecessary the recording bias magnetic field are proposed in Japanese laid open patent JP-A-1-241051, JP-4-192138 and JP-A-3-156751. In JP-A-1-241051, a recording medium using the demagnetizing field of the second magnetic layer (writing layer) without application of a external bias field is shown.
In JP-A-3-156751, a recording medium using the leaky magnetic field of the fourth magnetic layer (initializing layer) without application of an external bias field is shown. In JP-4-192138, a medium having a fifth magnetic layer having no exchange coupling with the fourth magnetic layer, and a recording method using the leaky magnetic field from the fifth magnetic layer without an external bias magnetic field is shown.
However, in magnetooptic recording, some media described above which reduce the necessary external recording bias magnetic field or make unnecessary the external recording bias magnetic field have the following problems:
In a recording medium using the opposing demagnetizing field of the second magnetic layer (writing layer) without application of an external bias field as described in JP-A-1-241051, it is necessary to change the composition of the writing layer substantially from the compensation composition, and to obtain a composition which shows a large value of saturation magnetization. But, if a material of such composition is chosen for the writing layer, the shapes of the domains which are formed fall into disorder, or multi-domains occur and there is a problem of not getting a good reproducing characteristic.
In a recording method using the leaky magnetic field from the fifth magnetic layer which is formed as a bias layer without an external bias magnetic field as described in JP-A-4-192138, if some domains are formed in the fifth magnetic layer during the laser irradiation used in recording, repeated overwriting brings about random domains and the fifth layer does not function as a bias layer. Therefore, making the Curie temperature of the fifth magnetic layer higher than that of the second magnetic layer, and not making the magnetization of the fifth magnetic layer reverse as a result of the laser irradiation used in recording is necessary. But, during the recording period, the irradiated part of the fifth magnetic layer remains magnetic. In this case if the direction of magnetization of the laser irradiated part is the same as that of magnetization of the peripheral region of the laser irradiated part, the leaky magnetic field from the peripheral region and the leaky magnetic field from the laser irradiated part oppose each other, and a large leaky magnetic field is not generated as shown in FIG. 1.
Therefore, to make the direction of magnetization of the laser irradiated part and that of magnetization of the peripheral region oppose each other, the compensation temperature of the fifth magnetic layer is adjusted in the vicinity of the Curie temperature of the writing layer. Then the temperature at the centre of the laser irradiated part becomes higher than the compensation temperature of the fifth magnetic layer during the recording period, and the magnetization of the laser irradiated part and its peripheral region which oppose each other, generate a leaky magnetic field from the peripheral region and a leaky magnetic field from the laser irradiated part which reinforce each other.
However, to provide the compensation temperature as described above, the saturation magnetization of the fifth magnetic layer at room temperature cannot have a large value. Using rare earth-transition metal magnetic films, the greater the composition of rare earth is than the compensation composition, the higher the compensation temperature is above room temperature, and at the same time the larger the saturation magnetization at room temperature. If the saturation magnetization becomes larger than a certain value, the compensation temperature exceeds the Curie temperature, the compensation temperature is ineffective. Therefore, in this medium a large leaky magnetic field is not generated from the peripheral region and the laser irradiated part as can be seen in FIG. 2.
In a recording method using the leaky magnetic field of the fourth magnetic layer (initializing layer) without an external bias magnetic field as shown in JP-A-3-156751 and so on, effectively large leaky magnetic fields cannot be generated as described above because the Curie temperature of the fourth magnetic layer is higher than that of the second magnetic layer (writing layer). Accordingly, one example showed that a leaky magnetic field was enlarged owing to an initializing layer having a thickness thicker than 80 nm. In this case, there was a problem of decrease of recording sensitivity.